The 1st Marquess was one of the few people to become a Marquess without inheriting any titles prior to the creation. He sat in the British House of Lords as one of the twenty-eight original Irish Representative Peers from 1800 to 1821. He was succeeded by his son from his first marriage to Lady Sarah Seymour. The 2nd Marquess, better known as Lord Castlereagh (he went by the courtesy title of Viscount Castlereagh from 1796 to 1821), was a noted statesman and diplomat. Castlereagh is best remembered for his tenure as Foreign Secretary of the United Kingdom from 1812 to 1822 and played an important role at the Congress of Vienna of 1814 to 1815. He committed suicide in 1822, one year after succeeding his father in the marquessate. The 2nd Marquess did not follow his father as a Representative Peer into the House of Lords, this permitted him to continue fulfilling his roles in the House of Commons.

Castlereagh was succeeded by his half-brother, the 3rd Marquess. He was the only son from the 1st Marquess's second marriage to Lady Frances Pratt, daughter of Charles Pratt, 1st Earl Camden. He was a General in the Army and like his elder half-brother a prominent politician and diplomat. Lord Londonderry served as Under-Secretary of State for War and the Colonies from 1807 to 1809, fought in the Napoleonic Wars and was Ambassador to Austria from 1814 to 1823. In 1814 he was created Baron Stewart, of Stewart's Court and Ballylawn in County Donegal, in the Peerage of the United Kingdom. In 1819 Londonderry married as his second wife Lady Frances Anne Vane-Tempest (died 1865), daughter and wealthy heiress of Sir Henry Vane-Tempest, 2nd Baronet, through which marriage substantial estates in County Durham came into the Stewart family. He assumed the additional surname of Vane on his marriage and in 1823 he was created Viscount Seaham, of Seaham in the County Palatine of Durham, and Earl Vane, with remainder to the male issue of his second marriage. These titles are in the Peerage of the United Kingdom.

He was succeeded by his son, the 7th Marquess, who was also a Conservative politician. He had a career in both Irish and British politics but is best known for his role as Secretary of State for Air from 1931 to 1935. Lord Londonderry subsequently gained notoriety for his informal diplomatic contacts with senior members of the German government. He made six visits to Nazi Germany between January 1936 and September 1938 meeting Adolf Hitler on a number of occasions and sympathising with some of his viewpoints.[3] His wife Edith Vane-Tempest-Stewart, Marchioness of Londonderry, was an influential society hostess remembered for her close friendship with Ramsay Macdonald. Lord Londonderry was succeeded by his son, the 8th Marquess. He represented County Down in the House of Commons as a Conservative from 1931 to 1945. As of 2014 the titles were held by his grandson, the 10th Marquess, who succeeded in 2012.

The heir apparent to the marquessate is styled Viscount Castlereagh, although the Marquess is also the Earl Vane, and the heir apparent's heir apparent (when such exists), in one of only two exceptions (the other being Duke of Manchester) to the 'rule' that the heirs of Viscounts do not use courtesy titles, is styled Lord Stewart.[citation needed]

Between 1823 and 1854 and between 1872 and 1999, the Marquesses of Londonderry sat in the House of Lords as The Earl Vane. Between 1822 and 1823 and between 1854 and 1872, they sat as The Lord Stewart.